1. Field of the Invention
This invention relates to a shift control system for an automatic transmission of a vehicle, and more particularly to a control system for controlling a so-called clutch-to-clutch shift in which one of two frictional engagement means such as a clutch, a brake and the like is released while the other frictional engagement means is applied.
2. Related Art
As is known in conventional automatic transmissions of vehicles, shifts between one gear stage and another gear stage are facilitated by engaging a one-way clutch for the gear stage to be set. However, since the automatic transmission having such a construction inevitably necessitates to incorporate the one-way clutch therein, there occurs an inconvenience that the automatic transmission becomes large in size. Therefore, the automatic transmission has been recently constructed by a gear shifting device only without the one-way clutch. In the automatic transmission of this type, it is required to conduct a so-called clutch-to-clutch shift, in which two frictional engagement means are simultaneously switched between engaged and disengaged states, to set a given gear stage.
In case the clutch-to-clutch shift is conducted, if the two frictional engagement means are both in a released condition, it is likely to cause undesirable racing of the engine. To the contrary, when both the engaging forces of these frictional engagement means becomes excessively large, there occurs a so-called tie-up condition, which results in considerably decreasing an output shaft torque. Moreover, in case rotational fluctuation is caused so slowly to prevent a shift shock, it takes a long time to accomplish the shift operation so that a so-called delayed response occurs.
Thus, the engaging forces of the frictional engagement means concerned in the clutch-to-clutch shift, give a considerable influence on the vehicle attitude, and must be therefore controlled while taking into account a mutual relation therebetween. In this regard, for example, JP-A-62-261,747 discloses a control system which is so constructed that, when the clutch-to-clutch shift is made, one frictional engagement means on an applied side is maintained in the condition able to engage immediately while the other frictional engagement means on a released side is controlled in its engaging force, whereby an input R.P.M. is caused to be varied at a first changing rate at an initial stage followed by a next stage in which the input R.P.M. is caused to be varied at a second changing rate, and a final stage in which the frictional engagement means on the applied side is completely engaged when the input R.P.M. reaches a synchronous revolution speed of the gear stage to be established after the shift.
In the control system disclosed in the JP-A-62-261,747, the control for smoothly changing the input R.P.M. is effected only by controlling the engaging force of the frictional engagement means on the released side. In consequence, the control of the input R.P.M. can be performed without delay as compared with the control system of such a type that both the frictional engagement means must be simultaneously controlled. In addition, since the input R.P.M. is caused to be varied smoothly, occurrence of the shift shock is improved.
In the aforementioned conventional control systems, only the frictional engagement means on the released side is substantially controlled upon the clutch-to-clutch shift. This is advantageous for preventing the delayed shift control. However, it is required to perform the control of the engaging force of the frictional engagement means on the released side by taking into consideration the change in the input R.P.M. In other words, the engaging force of the frictional engagement means on the released side must be controlled while detecting the change in the input R.P.M., so that the shift delay can be improved only in a limited manner.
Moreover, the aforementioned conventional control systems are so constructed that the engaging forces of the respective frictional engagement means are controlled only relative to the input R.P.M. Therefore, if it is intended to cause the input R.P.M. to be changed moderately so as to prevent the shift shock, a long period of time is required until the input R.P.M. reaches the synchronous revolution speed of the gear stage to be established after the shift. In consequence, the time required for the shift is not necessarily shortened to an enough level and the shift shock can be prevented only in a limited manner.